Since a polyacetal resin is excellent in a balance of rigidity, strength, toughness, anti-creep life, fatigue resistance, chemical resistance and sliding properties, heat resistance, and the like and is easy to process, it is used as an engineering plastic in wide range of applications including electric parts, mechanical parts of electric parts, automobile parts, and other mechanical parts as representatives.
In particular, as characteristic applications of the polyacetal resin, it is used for sliding-related parts such as gears and cams in mechanical parts of electric and electronic devices. Parts in sliding-related applications, such as gears and cams, are frequently used in a state where a certain load is applied to the parts. As properties, to say nothing of sliding properties, not only short-term mechanical properties such as rigidity, strength, and toughness but also long-term properties such as anti-creep life and fatigue resistance are frequently required.
Recently, in these parts in sliding-related applications and the like, weight saving, miniaturization, and increase in durability of the parts have been required. As a result, there has been desired a polyacetal resin composition wherein not only short-term mechanical properties such as rigidity, strength, and toughness but also long-term properties such as anti-creep life and fatigue resistance are improved.
Thus, as common methods, as in the case of other engineering plastics such as polyamides, improvement of its performance has been attempted by blending an inorganic filler such as glass fibers, wollastonite, or carbon fibers. However, in the case that glass fibers or an inorganic filler is blended into the polyacetal resin, the blending is effective for improvement of mechanical properties such as rigidity and toughness but sliding properties originally characteristic of the polyacetal resin and long-term properties such as anti-creep life and fatigue resistance and further toughness are sometimes severely deteriorated, so that the method is not effective. Moreover, in the case of blending the glass fibers or an inorganic filler in a large amount, thermal stability of the polyacetal resin is sometimes lowered and thus there is a possibility that moldability, heat-resistant aging property, or the like may be adversely effected, so that the method becomes problematic in some cases.
There have been known polyacetal resin compositions comprising a polyacetal resin and calcium carbonate wherein a saturated fatty acid, an unsaturated fatty acid, or a metal salt thereof is used as an interfacial adhesion accelerator between the polyacetal resin and calcium carbonate (e.g., Patent Documents 1 to 3) and a polyacetal resin composition wherein a specific sulfate compound is used as an interfacial adhesion accelerator (e.g., Patent Document 4). The polyacetal resin compositions are known to be excellent in a balance of rigidity and toughness and also excellent in thermal stability and sliding properties. Furthermore, polyacetal resin compositions comprising a polyacetal resin, calcium carbonate, and a specific fatty acid ester are known to be particularly excellent in sliding properties (e.g., Patent Documents 5 and 6).
The above polyacetal resin compositions comprising a polyacetal resin and calcium carbonate have characteristics that they are excellent in a balance of rigidity and toughness and also excellent in sliding properties and thermal stability as compared with the cases that glass fibers or an inorganic filler is blended into the polyacetal resin. However, in the case of the use as actual structural parts, there is a case that higher rigidity and toughness are required. Moreover, with regard to the increase in durability of the structural parts, long-term properties such as anti-creep life and fatigue resistance are sometimes insufficient.
In addition to the above problems, there is a case that improvement of durability of the polyacetal resin against an acid, i.e., acid resistance, is required. The acid resistance herein means resistance to weight loss and deterioration in mechanical properties that a molded article of the polyacetal resin suffers from, when being placed under an acidic atmosphere, through corrosion of the molded article per se. As a specific example, a fuel-feeding unit in automobile parts may be mentioned. Hitherto, the polyacetal resin has been used in large-sized parts which come into direct contact with gasoline, such as fuel pump modules, because of the properties of excellent chemical resistance, especially gasoline fuel impermeability. For the purpose of contributing to prevention of recent global warming, there is a trend of actively popularizing diesel fuel vehicles which less emit carbon dioxide instead of gasoline fuel vehicles and it is attempted to use the polyacetal resin in fuel-feeding units for diesel fuel. However, since light oil to be used as diesel fuel contains a high level of sulfur, i.e., a high level of sulfur oxides as compared with gasoline fuel and the maximum temperature for use is higher than the case of gasoline fuel, there is a problem that parts which come into direct contact with the diesel fuel, e.g., pump modules, fuel valves, fuel tank flanges, fuel level gauges, and the like may be corroded and cannot exhibit a sufficient durability as the parts in the case of conventional polyacetal resins.
For the purpose of solving such a problem, there have been disclosed a composition wherein a hindered phenol-based compound, a specific phosphorus-based stabilizer, a specific nitrogen compound, and a specific metal hydroxide or alkoxy metal are blended into a polyacetal resin (Patent Document 7), a material wherein an alkaline additive is incorporated into a polyacetal resin base material (Patent Document 8), and a composition comprising a polyacetal resin and also zinc oxide and polyalkylene glycol (Patent Document 9). As a result, there is a tendency that acid resistance may be improved as compared with conventional polyacetal resins but a balance of rigidity and toughness tends to be disrupted owing to the addition of various additives, so that a design with a large thickness is required due to insufficiency of rigidity and toughness in the case of the use as large-sized parts such as fuel pumps. Moreover, there is a problem that the above materials lack reliability because of insufficient long-term properties such as anti-creep life.
As described above, in various applications, there has been desired a polyacetal composition which is excellent in a balance of thermal stability and mechanical properties, such as rigidity and toughness, and also excellent in anti-creep life, fatigue resistance and further acid resistance.    Patent Document 1: British Patent Application Publication No. 1123358    Patent Document 2: JP-A-1-170641,    Patent Document 3: JP-T-2004-506772,    Patent Document 4: U.S. Pat. No. 4,456,710,    Patent Document 5: JP-A-1-263145,    Patent Document 6: JP-A-5-51514,    Patent Document 7: Japanese Patent No. 3157579,    Patent Document 8: JP-A-11-302497,    Patent Document 9: JP-A-2001-11284,